Field of the Invention
This invention relates generally to a vehicle structural member that includes a micro-truss reinforcement core and, more particularly, to a vehicle rocker panel that includes a micro-truss reinforcement core extending along the length of the rocker panel, where the core includes sections having varying densities so as to match the stiffness requirements of the panel at different locations with minimal mass.
Discussion of the Related Art
Modern vehicles are equipped with a number of impact beams and structural members providing structural integrity against collisions and impacts with objects, such as other vehicles. More particularly, impact beams are traditionally used in vehicle designs to protect occupants from front, side and/or rear impacts by absorbing energy through deformation in the event of a vehicle crash and distributing the applied dynamic loads to other energy absorbing sub-systems on the vehicle. For example, it is known to provide impact beams in a front energy management or bumper assembly, a rear energy management or bumper assembly and side impact beam assemblies on a vehicle. Vehicles also typically include rocker panels that extend along the bottom side edge of the vehicle. These rocker panels are typically rectangular steel tubes including spaced apart reinforcing walls or inserts that have a thickness and spacing depending on the stiffness and impact requirements for a particular location on the vehicle, where an increase in the thickness and number of the reinforcing walls increases the weight of the vehicle.
Impact beams at the front and rear of the vehicle are usually referred to as bumper beams, and impact beams on the sides of the vehicle are sometimes referred to as anti-intrusion bars. In all cases, it is desirable to provide an impact beam with low mass, high flexural stiffness and strength, and high energy absorption per unit mass. The lightweight requirement is predicated by fuel economy standards and the fact that impact beams are located both very close to and very far from the vehicle's center of mass. Maximizing the flexural stiffness and strength is necessary if the beam is to survive low speed impacts without damage and transfer impact loads throughout the duration of a high speed impact event. Further, a high level of energy absorption translates into reduced load transfer to the occupants of the vehicle, thus increasing safety.
It is known in the art to provide vehicle impact beams and structural members having sandwich structures. These prior art impact beams can generally be categorized into three designs, namely, hollow beams that are fully or partially reinforced with a polymer or metallic foam, single or dual-sided facesheets reinforced with a honeycomb-like cellular core, and formed composite impact beams. For hollow metallic or polymer matrix composite tube structures which are fully or partially reinforced with a lightweight foam core, the material used for the core can be either a metallic or polymeric foam that is bonded, mechanically attached or interference fit into the tube structure. The purpose of the core is to carry shear loads in the sandwich structures and absorb energy in the event of a low or high speed impact, which is a distinction dependent on the density and composition of the foam. The use of honeycomb or honeycomb-like ordered cellular cores to provide reinforcement to one or two flat facesheets have an open-sided sandwich designs and have honeycomb, discrete-stiffened or wine-crate structures extending from the front face of the impact beam back towards the passenger compartment of the vehicle. If a second facesheet is not included between the core and the passenger compartment, then the core material must be relatively dense since it provides most of the flexural stiffness to the structure adjacent to the shear load transfer.
For continuous or discontinuous fiber reinforced polymer matrix composite impact beams, the matrix material may either be a thermoplastic or thermosetting polymer introduced via resin transfer molding, compression molding, blow molding, or other similar fabrication processes.
It is also known in the art to fabricate a three-dimensional network of photopolymer waveguides comprising a unitary truss or lattice architecture, hereafter referred generally as a micro-truss structure or micro-truss core. For example, U.S. Pat. Nos. 7,653,279 and 7,382,959 disclose a process for fabricating such a micro-truss structure. Generally, the process includes providing a reservoir or mold filled with a volume of a curable monomer and covered by a mask including strategically positioned apertures. UV light sources are positioned relative to the mask and exposure to collimated UV light through the mask apertures forms a series of interconnected self-propagating photopolymer waveguides, referred to herein as struts, to form the truss or lattice architecture. Once the photopolymer waveguides are formed, the reservoir is emptied of the unpolymerized monomer which was not exposed to UV light. The micro-truss structure may then undergo a post-cure operation to increase the cross-link density in the photopolymer waveguides. This post-cure may be accomplished via a thermal cure, an additional exposure to UV light, an equivalent method or combinations thereof.